I think an educational game should actually let you know what you are learning. As an adult I remember some old games, or old songs that I learnt when i was a kid. Then I learn something as an adult and I go "Oh thats what they were talking about" cause they didn't give me the context. I found a bunch of good educational games at this site http://www.gamestreamer.com/

I found a lot of education games at http://www.gamestreamer.com/ They have lots of different types of games that I get from there, but I torture my kids with the educational ones.

One thing I always liked about the valve games is that I never had to repurchase any game after I had bought it. One thing I disliked about steam was the fact that its all the same type of game. I found another site since then that I use http://games.gamestreamer.net/ It allows the same kind of thing, seems to have much better speeds as well. Steam always has such a limited genre in game selection.